There are many instances in which it may be desirable to analyze all the information contained in a very broadband signal, such as a microwave signal. Limitations on processing speed often dictate an approach that entails dividing the frequency band of the signal into a number of narrower frequency sub-band signals or channels (in a process sometimes referred to as channelizing), and processing the reduced amount of information in each of these narrower sub-bands. The processing speed acceptable for processing the information in each sub-band or channel decreases as the signal is divided into narrower sub-bands.
RF domain channelizing techniques are well known in the art and tend to require relatively bulky, highly power-consumptive equipment. Using currently available equipment and techniques, a microwave system for dividing a 50 GHz wide signal into one hundred 0.5 Ghz wide sub-bands would typically require tens of Watts of power and several cubic feet of volume. Existing optical techniques (e.g. Wavelength Division Multiplexing, or WDM) offer considerable savings in weight, size and power over microwave approaches, but are typically not adept at resolving frequency bands that are spaced less than many GHz apart and currently, the best commercially-available devices can separate frequencies that are a minimum of 50 GHz apart.
In WDM, optical signals are routed by discriminating between the wavelengths and information is impressed on an optical carrier signal (i.e. a light wave) by detecting and modulating the intensity (not amplitude) of the light wave (often referred to as Intensity Modulation Direct Detection, or IMDD). This approach is relatively easy to implement, but does not allow for efficient frequency routing because information that is impressed on the carrier signal by modulating the intensity of the carrier signal can be routed by wavelength only if all of the information impressed upon the carrier signal is routed with that wavelength. Thus, for example, conventional WDM technology (IMDD) may be applied to routing a series of wavelengths (i.e. sub-bands or channels) separated by 50 GHz and each carrying 10 GHz of information (i.e., the optical bandwidth of the carrier plus signal is approximately 20 GHz) but not to routing a carrier with 50 GHz of impressed information in 1 GHz channels. This is due to the fact that the optical sidebands produced by Intensity Modulation are not the same as those produced by Amplitude Modulation. For this reason, an optical sub-band of an Intensity-Modulated optical carrier signal would not, on Direct Detection, produce the information that was originally in the baseband-equivalent of that sub-band.
U.S. Pat. No. 6,094,285 discloses an optical RF channelizer system that employs Bragg diffraction gratings to spatially split an optical carrier signal into sub-bands, and can further employ Fabry-Perot filters tuned to specific sub-bands of the diffraction gratings. Because the system disclosed employs concatenated polarizing beam splitters, the light polarization must be accurately maintained throughout the system. These are relatively costly devices and are very difficult to implement at the microchip level with semiconductor devices. Fabry-Perot filters additionally require large cavities to achieve a high quality factor Q, and are thereby also difficult to implement at the microchip level. Furthermore, polarizing beam splitters are not very efficient, and typically suffer losses on the order of 1 dB or more, thereby limiting a system according to this patent to approximately 10 channels.
Other types of fiber-optic filters that are able to filter IMDD information correctly are currently available. These transversal or delay-line filters are relatively large for on-chip applications and furthermore are inherently inefficient because they are designed to discard out-of-band information rather than channel it to its proper destination.
What is needed is a method and apparatus for dividing a wide-band signal into a large number of smaller frequency bands that is power efficient, not vulnerable to acoustic noise effects, and amenable to on-chip implementation with semiconductor devices. The embodiments of the present disclosure answer these and other needs.